Instant tube heaters are known in the field of beverage dispensers such as water dispensers or coffee capsule machine and the like. The advantage consists in heating quickly, repeatedly and economically liquid on demand, for example, for brewing a beverage from a single dose of beverage ingredients contained in a capsule or pod. The heater tube is economical since it essentially heats only the volume of liquid required as it traverses the heating tube. The tube heater has a low mass of inertia and does not need any significant pre-heating time. Therefore, the preparation of a hot beverage is speedier and requires less energy compared to a boiler or a thermo-block.
U.S. Pat. No. 4,975,559 relates to a device for heating and aerating water in a coffee machine. The water circuit has water circulation channels and air retaining pockets disposed at intervals along the water circulation channels above the normal water levels of the channels. The air retaining pockets provide turbulence in circulating water and cause the air retained in the pockets to be dissolved in the water.
US2006027103A1 relates to a device for heating liquid in a beverage machine. The device comprises a tube heater with a water inlet, a water outlet and an insert inside the tube comprising helical grooves. The water is forced through a small gap in helical manner. One problem is that water temperature is difficult to control and can be over-heated. This requires a complex set of resistors which are electrically linked. Furthermore, the inside insert provides an elevated pressure loss in the fluid circuit that needs to be overcome.
In particular, an instant tube heater can comprise a glass tube coated with an electro-thermal film. For example, Chinese utility model CN202636656 (U) provides a heating body assembly for instant water dispensers and water boilers. The heating body assembly comprises a water inlet end and a water outlet end, wherein a heating tube is arranged between the water inlet end and the water outlet end. The heating body assembly is characterized in that the heating tube is a single tube having an outer diameter of 20-25 mm, a length of 180-230 mm and a power of 1800-2800 W.
Generally, the temperature regulation is obtained by the control unit of the beverage machine sensing the temperature with a NTC sensor directly at the water flow outlet and powering the heating tube on and off accordingly.
It has been found that the temperature in the heating tube is not homogeneous but forms a gradient from the centerline of the tube towards its periphery. Generally, since the liquid flow is laminar, the liquid flowing in the centre of the tube is cooler than the liquid flowing adjacent the tubular wall of the tube. Since the NTC sensor preferentially measures the temperature coming from the centre of the tube, the temperature regulation is poor since the measured temperature is systematically too low. Moving the NTC sensor relative to the tube (towards the hotter region of the temperature gradient) does not provide a better regulation.
In addition, since the sensed temperature is lower than the real average temperature of liquid, the tube tends to be over-heated which thus creates a cushion of steam between the heating tube and the liquid inside. This small insulation area reduces drastically the heat efficiency of the tube heater.